The heat pipe is a sealed element in which heat is transferred by evaporation, vapor movement and condensation. The condensate is then returned to the evaporator by capillary action in a wick. The first such devices used homogeneous wick structures of uniform cross section and porosity. The heat transfer rate in these heat pipes was limited by the high viscous drag due to liquid flow through the small, convoluted passages in the wick. It was subsequently shown that greater heat transfer over greater heat pipe lengths could be obtained through the use of a composite wick having a controlled inhomogeneity. Structures having wicks which cover grooves, slabs, and arteries cut into the casing have been demonstrated. These composite wicks have a small pore surface at the liquid/vapor interface in the evaporator region, liquid passages of large cross section and minimum convolution through all or most of the length of the heat pipe from condenser to evaporator. The small pores provide high capillary pressures for maximum pumping. The large, longitudinal passages permit the flow of large quantities of liquid with low pressure loss.
The highest axial power density reported to date has been obtained with a composite wick consisting of longitudinal grooves covered with woven screen having 508.times.3600 wires per inch. The grooves were reported as 0.06.times.0.05 cm, the screen wire diameters as 0.0025 cm and 0.0015 cm respectively, and the effective pore size as 0.0011 cm. Two layers of screen were used so that the apparent thickness of the screen wick was that of two warp wires plus two woof wires, or 0.008 cm (0.003"). For proper utilization of the pumping pressure afforded by the very fine pore screen, the liquid/vapor interface in the evaporator must fall within the screen. Since the two layers of screen are very thin, the tolerance on liquid level is only 0.008 cm. All variation due to gravitational orientation, liquid inventory, power level and thermal expansion of the liquid must cause a change in liquid level of less than 0.008 cm, an extremely difficult control task to manage in quantity production. Moreover, it has proven difficult to produce longitudinal grooves in the inside walls of refractory metals such as molybdenum and tungsten. This difficulty has prevented the extension of this high performance construction technique to high temperature heat pipes such as might be used for thermionic out-of-core nuclear space power systems.
Although small pore size heat pipe wicks have also been constructed using the sintering of copper and nickel powders, such wicks were of the homogeneous type and not suited for long liquid flow paths, because of the high liquid flow pressure loss through the small pores. Such wicks have limited liquid flow lengths and thus limit the working length of the heat pipe to only about one-half inch.